Azine liquid crystal compounds for use in light-control devices

ABSTRACT

A light-control device includes a liquid crystal material sandwiched between a pair of planar, light-transmitting members. The liquid crystal material particularly includes an asymmetrical azine compound having the general formula: ##STR1## wherein R 1  and R 2  are different para-substituents selected from the group comprising cyano, halogen, alkyl, and substituted alkyl radicals in which the alkyl radical has from one to nine carbon atoms, and wherein X and Y are selected from the group comprising hydrogen and methyl radicals.

This is a continuation-in-part of our application Ser. No. 937,507 filedAug. 28, 1978, now abandoned and entitled "Azine Liquid CrystalCompounds For Use In Light-Control Devices" which in turn was acontinuation-in-part of our application Ser. No. 861,954 filed Dec. 19,1977, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to the art of visual displays andsimilar light-control devices and relates more particularly to liquidcrystal displays which incorporate an azine compound as part of thesystem for selective transmission of visible light.

Organic substances which exhibit a mesophase have been known for manyyears, but it has been only more recently that the technology of liquidcrystal materials has been developed sufficiently to achieve commercialapplication in such devices as wrist watches and digital readouts.

The substances which exhibit a liquid crystal phase, as postulated byGray and Harrison in U.S. Pat. No. 3,947,374 for example, comprise amolecule with a central linkage group and a pair of distal chemicalgroups of varying character. These patentees recognize the prior utilityof Schiff Bases and themselves disclose commercially useful liquidcrystal properties for certain byphenyl compounds. Gray and Harrisonalso teach that the presence of an unsaturated group in the linkage unitis associated with undesirable, chemical and/or photochemicalinstability. In addition, Kmetz and Willisen in "NonemissiveElectrooptic Displays," Plenum Press, New York and London (1976), havesuggested possible display performance for azine compounds withidentical distal groups, although the authors admit that almost nothingis known about the properties of liquid crystal azine compounds.

SUMMARY OF THE INVENTION

Contrary to the teachings and suggestions of the prior art, applicantshave discovered that certain azine compounds, despite the presence oftwo double bonds in the central linkage group, are adequately stable andhave other useful properties for display devices. In particular, theazine compounds of the present invention have dissimilar distal groups,i.e. the azine molecule is asymmetric.

Accordingly, a general object of the present invention has been toprovide a new class of liquid crystal compounds which have unexpectedutility in a variety of light-control devices.

Another object of the present invention is to provide a new class ofliquid crystal compounds which are stable and which are easily andeconomically synthesized.

Applicants have established that a liquid crystal compound which isuseful for visual display purposes possesses a melting point of nohigher than about 75° C., has a comparatively low viscosity in order toexhibit a short turn-off time, is chemically stable and non-toxic, andshows a high transition temperature from the mesomorphic phase to theisotropic liquid state of at least about 50° C. In order to formulate amaterial that is acceptable for a given display application, it issometimes necessary to mix a particular asymmetrical azine compound ofthe present invention with another like compound or with some othersubstance, especially to obtain a desired melting point, it having beenfound that mixtures of some liquid crystal materials have melting pointswhich are depressed to a considerably greater extent than is predictablefrom simple thermodynamics. On the other hand, the temperatures oftransition to isotropic liquid ordinarily vary linearly with molaraverage composition, between the respective temperatures for the purecomponents of the mixture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The particular azine compounds of the present invention have the generalformula: ##STR2## wherein R₁ and R₂ are different para-substituentswhereby to produce an asymmetrical molecular structure, wherein R₁ andR₂ are selected from the group comprising cyano, halogen, alkyl, andsubstituted alkyl radicals in which the alkyl radical has from one tonine carbon atoms, and wherein X and Y are selected from the groupcomprising hydrogen and methyl radicals. The transition temperatures forspecific compounds answering this definition and comprising examples ofasymmetrical azines which we have specifically synthesized, are setforth in Table I below.

                  TABLE I                                                         ______________________________________                                        Transition Temperatures For                                                   Selected, Asymmetrical Azine Compounds                                                                      Crystal (or                                                                            Nematic to                                                           Smectic) to                                                                            Isotropic                                                            Nematic  Liquid                                 R.sub.1  R.sub.2  X      Y    Transition                                                                             Transition                             ______________________________________                                        F        n-C.sub.3 H.sub.7                                                                      CH.sub.3                                                                             H    (Sm) 69° C.                                                                         75° C.                      CN       n-C.sub.4 H.sub.9                                                                      CH.sub.3                                                                             H    (Sm) 56    117                                  CN       n-C.sub.5 H.sub.11                                                                     H      CH.sub.3  71    131                                  CH.sub.3 n-C.sub.4 H.sub.9                                                                      H      H         58    87                                   CH.sub.3 n-C.sub.5 H.sub.11                                                                     H      H         60    100                                  CH.sub.3 n-C.sub.7 H.sub.15                                                                     H      H    (Sm) 56    89                                   CH.sub.3 n-C.sub.8 H.sub.17                                                                     H      H         62    85                                   CH.sub.3 n-C.sub.9 H.sub.19                                                                     H      H         64    88                                   C.sub.2 H.sub.5                                                                        n-C.sub.4 H.sub.9                                                                      H      H         39    82                                   C.sub.2 H.sub.5                                                                        n-C.sub.5 H.sub.11                                                                     H      H         32    92                                   C.sub.2 H.sub.5                                                                        n-C.sub.6 H.sub.13                                                                     H      H         37    82                                   C.sub.2 H.sub.5                                                                        n-C.sub.6 H.sub.13                                                                     CH.sub.3                                                                             H         32    62                                   C.sub.2 H.sub.5                                                                        n-C.sub.8 H.sub.17                                                                     H      H         33    75                                   n-C.sub.3 H.sub.7                                                                      n-C.sub.4 H.sub.9                                                                      H      H         37    95                                   n-C.sub.3 H.sub.7                                                                      n-C.sub.4 H.sub.9                                                                      CH.sub.3                                                                             H         39    76                                   n-C.sub.3 H.sub.7                                                                      n-C.sub.4 H.sub.9                                                                      H      CH.sub.3  44    77                                   n-C.sub.3 H.sub.7                                                                      n-C.sub.5 H.sub.11                                                                     H      H    (Sm) 34    105                                  n-C.sub.3 H.sub.7                                                                      n-C.sub.5 H.sub.11                                                                     H      CH.sub.3  43    83                                   n-C.sub.3 H.sub.7                                                                      n-C.sub.6 H.sub.13                                                                     H      H         23    92                                   n-C.sub.3 H.sub.7                                                                      n-C.sub.7 H.sub.15                                                                     H      H         37    95                                   n-C.sub.3 H.sub.7                                                                      n-C.sub.8 H.sub.17                                                                     H      H    (Sm) 28    89                                   n-C.sub.3 H.sub.7                                                                      n-C.sub.9 H.sub.19                                                                     H      H    (Sm) 42    93                                   n-C.sub.4 H.sub.9                                                                      n-C.sub.6 H.sub.13                                                                     H      H    (Sm) 38    86                                   n-C.sub.4 H.sub.9                                                                      n-C.sub.7 H.sub.15                                                                     H      H         34    91                                   n-C.sub.4 H.sub.9                                                                      n-C.sub.8 H.sub.17                                                                     H      H    (Sm) 33    84                                   n-C.sub.5 H.sub.11                                                                     n-C.sub.7 H.sub.15                                                                     H      H         48    98                                   n-C.sub.5 H.sub.11                                                                     n-C.sub.8 H.sub.17                                                                     H      H         43    92                                   CH.sub.2 -CH.sub.2 -CN                                                                 n-C.sub.4 H.sub.9                                                                      H      H         71    105                                  n-C.sub.5 H.sub.11                                                                     n-C.sub.9 H.sub.19                                                                     H      H         54    94                                   n-C.sub.6 H.sub.13                                                                     n-C.sub.7 H.sub.15                                                                     H      H         47    86                                   n-C.sub.6 H.sub.13                                                                     n-C.sub.8 H.sub.17                                                                     H      H         37    84                                   n-C.sub.6 H.sub.13                                                                     n-C.sub.9 H.sub.19                                                                     H      H         59    87                                   ______________________________________                                    

It will be noted that all of the compounds in Table I have meltingpoints (crystalline to smectic or to nematic transition point) below thedesired temperature of 75° C.; and in fact, many of them are below 30°C. Moreover, the clearing points (nematic to isotropic liquid transitionpoints) are in excess of 50° C. and, in many cases, above 80° C. Bycomparison, a symmetrical azine composition in which both R₁ and R₂ arealkyl radicals containing eight carbon atoms, the smectic to nematictransition temperature has been observed to be 68° C. with a nematicrange of only 20° C.

The asymmetrical azine compounds of the present invention are alsocharacterized by desirably low viscosity, expecially when formulated inmixtures for optical display purposes. Fast response or turn-off timesresult, as much as twice as fast as mixtures of ester liquid crystalcompounds. For example, a display with an ester liquid crystal mixtureshowed a turn-off of about 140 microseconds, whereas a correspondingdisplay employing a mixture of asymmetrical azine compounds according tothe present invention exhibited a turn-off time of about 60microseconds. Thus, the asymmetrical azine compounds of the presentinvention exhibit viscosities and turn-off times equivalent tocommercial biphenyl liquid crystal compounds while, at the same time,possessing higher and more desirable isotropic transition temperatures.In addition, the asymmetrical azine compounds of the present inventionexhibit improved chemical and photochemical stability over Schiff Baseliquid crystal compounds, although not as great as biphenyl or esterliquid crystal compounds.

The asymmetrical azine compounds of the present invention can becombined in mixtures with other liquid crystal compounds as well as withother asymmetrical azine compounds using conventional methods such thatthe resulting composition exhibits a greater mesomorphic range than thatof the individual compounds. In addition, asymmetrical azine compoundsof the present invention, expecially those having melting points of 60°C. or higher, can be usefully combined with biphenyl liquid crystalcompounds or with Schiff Base liquid crystal compounds or with esterliquid crystal compounds to elevate the transition temperature from thenematic phase to the isotropic liquid phase of the resultant mixture.The composition of one eminently useful mixture, according to thepresent invention is set forth in Table II below.

                  TABLE II                                                        ______________________________________                                        MIXTURE #1                                                                    Compound                                                                      R        R.sub.1   X        Y     % by Weight                                 ______________________________________                                        C.sub.3 H.sub.7                                                                        C.sub.6 H.sub.13                                                                        H        H     14.3                                        C.sub.3 H.sub.7                                                                        C.sub.5 H.sub.11                                                                        H        H     19                                          C.sub.4 H.sub.9                                                                        C.sub.6 H.sub.13                                                                        H        H     14.3                                        C.sub.1 H.sub.3                                                                        C.sub.4 H.sub.9                                                                         H        H     9.5                                         C.sub.1 H.sub.3                                                                        C.sub.8 H.sub.17                                                                        H        H     9.5                                         C.sub.3 H.sub.7                                                                        C.sub.8 H.sub.17                                                                        H        H     14.3                                        F        C.sub.3 H.sub.7                                                                         CH.sub.3 H     14.3                                        CN       C.sub.4 H.sub.9                                                                         CH.sub.3 H     4.8                                         ______________________________________                                         The nematic range of Mixture #1 is from -10° C. to 85° C.  

A twisted nematic display requires the use of a liquid crystal mixturehaving a positive dielectric anisotropy; and for prior art liquidcrystal compounds, this is ordinarily accomplished by utilizing a cyano(CN) group as one of the substituents, thus introducing a dipole alongthe long axis of the molecule. However, the asymmetrical azine compoundsof the present invention inherently possess a mildly positive dielectricanistropy without need to resort to synthesis so as to incorporate acyano group. However, when a display device with a low voltage thresholdis to be fabricated, a cyano group is advantageously employed as one ofthe para-substituenst in the azine compounds of the present invention;and as will be noted, Mixture #1 contains such a compound. A twistednematic display using Mixture #1 has displayed a threshold voltage of2.8 volts. In addition to compositions employing two or moreasymmetrical azine compounds, other compositions can be formulated usingother types of liquid crystal material. For example, ten percent byweight of a cyano ester, specifically cyano-phenyl-pentyl benzoate, maybe added to Mixture #1; and this latter composition has exhibited anematic range of from -22° C. to 83° C. In a twisted nematic display, itexhibits a threshold voltage of 1.60.

The azine liquid crystal compounds of the present invention exhibitutility in a variety of mixtures with other types of liquid crystalmaterials. For instance, display devices behaving other than as atwisted nematic display and incorporating the instant azine compoundsare described in Examples Nos. 1 and 7-9 below; and the instant azinecompounds may be combined advantageously with a variety of liquidcrystal materials of different classes as is set forth in all ofExamples Nos. 1-9:

EXAMPLE NO. 1

A liquid crystal mixture having a negative dielectric anisotropy wasmade using 60% p-methoxybenzylidene-p-butylaniline (MBBA) and 40%p-ethoxybenzylidene-p'-butylaniline (EBBA). Both of these liquid crystalmaterials are available from a subsidiary of 3M Company, Vari-LightCorporation. This mixture had a nematic to isotropic transitiontemperature of 58.0° C. A cell was made using this mixture; and anelectric field was applied across the electrodes. The cell exhibitedproperties of dynamic scattering.

Approximately 15% 4-n-propyl-4'-n-hexylbenzalazine was added to aportion of the foregoing MBBA-EBBA mixture. The new mixture exhibited anematic to isotripic transition of temperature of 63.1° C. Again, a cellwas fabricated using this latter mixture. An electric field was appliedacross the electrodes; and the mixture behaved as a negative liquidcrystal, i.e. dynamic scattering was evident.

EXAMPLE NO. 2

A liquid crystal mixture was made using a material from BDH Chemical,Ltd. (Dorset, England) known as E-7 biphenyl. This material is aeutectic mixture of 51% 4-cyano-4'-n'pentylbiphenyl; 25%4-cyano-4'-n-heptylbiphenyl; 16% 4-cyano-4'-n'octyloxybiphenyl; and 8%4-cyano-4'pentyl-p-terphenyl. The E-7 biphenyl mixture is reported tohave temperature range of -9° C. to +59° C. (G. W. Gray, Advances inLiquid Crystal Materials for Application, 1978 BDH publication).

A cell was made using a mixture of E-7 and 10%4-n-propyl-4'-n-hexylbenzalazine; and its temperature range was found tobe -10° C. to +66° C. In the twisted nematic cell, the electroopticthreshold was found to be 1.20 V rms as compared to E-7 alone which hada threshold of 1.15 V rms. The temperature range was thus significantlyimproved by the addition of the asymmetrical azine compound.

EXAMPLE NO. 3

A liquid crystal mixture was made using 28.2%trans-4-propyl-(4-cyanophenyl) cyclohexane; 42.4%trans-4-pentyl-(4-cyanophenyl)-cyclohexane; and 29.4%trans-4-heptyl-(4cyanophenyl)-cyclohexane. These phenylcyclohexanematerials are available from E. Mark (Darmstadt, Germany). The abovemixture showed a temperature range of -30° C. to +55° C. and anelectrooptic threshold of 1.10 V rms.

To the above phenylcyclohexane mixture was added 40%4-n-propyl-4'-hexylbenzalazine. In a twisted nematic cell, the mixtureshowed a temperature range of -50° C. to +73° C. and an electroopticthreshold of 1.30 V rms.

EXAMPLE NO. 4

A pyrimidine liquid crystal mixture was made using 40%5-n-pentyl-2-(4-cyanophenyl)-pyrimidine and 60%5-n-heptyl-2-(4-cyanophenyl)-pyrimidine. The above materials areavailable from Hoffmann-La Roche and Co., Ltd. (Basle, Switzerland). Atwisted nematic cell was made using the above mixture with 10%4-n-propyl-4'-n-hexylbenzalazine added. This latter mixture showed atemperature range of +30° C. to +51° C. and an electrooptic threshold of0.85 V rms.

EXAMPLE NO. 5

A mixture containing asymmetrical alkyl benzalazines was prepared tomeasure the affect of cyanobiphenyl liquid crystals on the dielectricanisotrophy, i.e. the cyano and halide substituted benzalazines were notused. The mixture consisted of:

    ______________________________________                                        17.7%         4-n-propyl-4'-n-hexylbenzalazine                                23.5%         4-n-pentyl-4'-n-propylbenzalazine                               17.7%         4-n-propyl-4'-n-octylbenzelazine                                17.7%         4-n-butyl-4'-n-hexylbenzalazine                                 11.7%         4-n-hexyl-4'-n-methylbenzalazine                                11.7%         4-n-methyl-4'-n-octylbenzalazine                                ______________________________________                                    

A sample cell of the twisted nematic type was made using the abovemixture and it exhibited an electrooptical threshold of 6.2 V rms.

To a portion of the above azine liquid crystal mixture was added 5% of amaterial from BDH Chemical, Ltd., known as K-18 liquid crystal(4-cyano-4'-n-hexylbiphenyl). A sample cell, of the twisted nematictype, was made using this new mixture; and it exhibited anelectrooptical threshold of 4.0 V rms. The combination of the K-18material and the asymmetrical azine showed an improvement in thepositive dielectric anisotropy.

EXAMPLE NO. 6

To the liquid crystal azine of Mixture #1 set forth hereinabove, therewas added 0.5% each of Sudan Black, Sudan IV, and β-carotene and 2%cholesteryl nonanoate, as in U.S. Pat. No. 4,032,219 granted to J.Constant, I. Shanks and E. Raynes. A sample cell of the twisted nematictype was made using the resultant mixture; and 9 V rms voltage wasapplied across the electrodes. The display exhibited properties ofsimilar displays as disclosed in U.S. Pat. No. 4,032,219.

EXAMPLE NO. 7

A sample cell was made using the same mixture as in Example #6, exceptthat 5% cholesteryl nonanoate was added instead of 2%. This material,when activated by an appropriate electric field, behaved typically aswould similar devices using the guest-host mechanism.

A sample cell using the mixture of this Example was made usingparallel-parallel orientation; and the resultant device operatedtypically as would devices constructed in a similar manner.

A sample cell using the mixture of this Example was made usingnormal-normal orientation; and this device operated typically as woulddevices made in a similar manner.

EXAMPLE NO. 8

An asymmetrical azine liquid crystal mixture, Mixture #1 hereinabove,was made with the addition of 1% BDH D-77*[1,5-(4'-isopropyl)-anilino-anthraquinone], 1% BDH-D-5* [1-(4'-n-butyl)anilino-4-hydroxyanthraquinone] and 0.5% cholesteryl nonanoate. Samplecells were made up with twisted, parallel-parallel, and normal-normalorientation; and when an appropriate electric field was applied to thedevices, they behaved in a manner typical of those described in thereference*.

EXAMPLE NO. 9

An asymmetrical azine liquid crystal mixture, Mixture #1 hereinabove,was made with the addition of 5% cholesteryl nonanoate, 2.5% BHD D-37*[1,5-(4'-n-butyl) anilinoanthroquinone], and 0.5% BHD D-77*[1,5-(4'-isopropyl)-anilinoanthraquinone]. Sample cells were made in thetwisted, parallel-parallel, and normal-normal orientation; and when anappropriate electric field was applied, the devices behaved typically aswould similar devices of this general type.

The asymmetrical azine liquid crystal compounds of the present inventionalso show a very low order of birefringence; and thus, digital and otherdisplays can be fabricated which are thin and possessed of fast responsetimes without undesirable color variations such as mottling.

Although the classical preparation of a symmetrical azine compoundtypically involves reaction of a hydrazine with an excess of analdehyde, the asymmetrical azines of the present invention areadvantageously synthesized by first preparing the hydrazone of one ofthe components by use of the procedure of G. R. Newkome and D. L. Fishel(J. Org. Chem., 31, 677 [1966]). The reaction then proceeds as in thefollowing typical equation: ##STR3##

In the foregoing reaction, the hydrazone (A) which has been specificallyprepared from n-propylbenzaldehyde, is mechanically mixed with themolecular equivalent weight of (B), n-pentylbenzaldehyde, in a suitablesolvent such as methanol, for a period of about 15-30 minutes. Theresulting azine product is then separated from the reaction mixture andpurified by recrystallization.

In order that the principles of the present invention may be morereadily understood, a physical embodiment thereof, applied to alight-control device or display, is shown in the accompanying drawingwherein FIG. 1 is a schematic, cross-sectional view showing anelectrooptical display incorporating an asymmetrical azine liquidcrystal composition according to the present invention.

Referring in detail to the drawing, a light-control device or shutterindicated generally by the reference numeral 10 is shown to comprise afirst and a second planar light-transmitting member, 12 and 14. A thinfilm of liquid crystal material 16 is confined between the planarmembers 12 and 14; and in order to retain the liquid crystal material 16in place, continuous liquid-stop ring 18 encompasses the liquid crystalmaterial between the planar members 12 and 14. As will be appreciated,the ring 18 may be configurated to define the perimeter of the numericaldigit or other symbol which it is desired to illuminate selectively.

In order to provide external electrical connection to the liquid crystalmaterial 16, the planar members 12 and 14, which are advantageouslyfabricated from a suitable glass, are coated on their confrontingsurfaces with thin conductive films 20 and 22 respectively. These filmsare advantageously fabricated of tin oxide and form respectiveelectrodes for suitable attachment to an external electrical circuit 24.Circuit 24 comprises a direct current source 26, such as a battery, anda manual or other switching device 28. As will be appreciated, theplanar members 12 and 14 are outwardly offset from the liquid stop ring18 in order to facilitate connections with the circuit 24.

According to conventional practice, a polarizing filter 30 is applied tothe outer plane surface of member 14. Similarly, a coating 32 is appliedto the outer planar surface of member 12; and the coating 32 may be acooperating polarizing filter with a reflective material such asmetallic silver depending on whether it is desired that the device betransmissive or reflective. In addition, a light source 34 is focused onthe polarizing filter 30; and in the case where the device 10 isreflective in nature, a viewing element 36, such as a photocell or thehuman eye, is disposed to observe the device 10 from the same aspect aslight source 34.

In accordance with the present invention, the liquid crystal material 16includes a compound which has a transition temperature of at least about70° C., which has a melting point of no higher than about 60° C., andwhich comprises an asymmetrical azine liquid crystal compound asdescribed hereinabove. Advantageously, the liquid crystal material 16may take the form of Mixture #1 which has been described hereinabove.

In operation, when the switching device 28 is directed into acircuit-opening condition, the viewing element 36 will contemplate abright background surrounding an opaque image of the indicia or otherpattern created by the liquid crystal material 16. Contrariwise, whenthe switching device 28 is directed into a condition completing thecircuit 24, the liquid crystal material 16 becomes light-transmissive,and the entire format of the device 10 appears reflective.

Various arrangements of light and dark areas may, of course, be createdas circumstances make desirable. The term "light-control device" as usedherein is intended to mean an optical gate for permitting or obstructingthe passage of light.

The drawing and the foregoing disclosures are not intended to representthe only forms of our invention in regard to the details of fabricationand manner of application. Changes in the construction of displaydevices and in the formulation of liquid crystal compositions, as wellas the substitution of equivalents, are contemplated as circumstancesmay suggest or render expedient; and although specific terms have beenemployed, they are intended in a generic and descriptive sense only andnot for the purposes of limitation, the scope of the invention beingdelineated in the following claims.

The following is claimed as invention:
 1. A light-control devicecomprising: first and second planar light-transmitting members; liquidcrystal material confined between said members which comprises at leastone nematic azine compound having a transition temperature from itsnematic phase to its isotropic liquid phase of at least about 50° C. anda melting point from its crystalline phase to a liquid crystal phase ofno higher than about 75° C., said nematic azine compound having thegeneral formula: ##STR4## wherein R₁ and R₂ are differentpara-substituents whereby to produce an asymmetrical molecularstructure, wherein R₁ and R₂ are selected from the group comprisingcyano, halogen, alkyl, and substituted alkyl radicals in which the alkylradical has from one to nine carbon atoms, and wherein X and Y areselected from the group comprising hydrogen and methyl radicals; andelectrode means disposed in contact with said liquid crystal material.2. A light-control device according to claim 1 wherein said liquidcrystal material comprises two or more said azine compounds havingasymmetrical molecular structure.
 3. A light control device according toclaim 1 wherein one of R₁ and R₂ is a cyano group.
 4. A light-controldevice according to claim 1 wherein said liquid crystal material furtherincludes a p'-cyanophenyl-p-alkylbenzoate.
 5. A light-control deviceaccording to claim 4 wherein said p'-cyanophenyl-p-alkylbenzoate isp'-cyanophenyl-p-n-pentylbenzoate.